Mineral oil lubricants are derived from various crude oil stocks by a variety of refining processes directed towards obtaining a lubricant base stock of suitable boiling point, viscosity, viscosity index (VI) and other characteristics. Generally, the base stock will be produced from the crude oil by distillation of the crude in atmospheric and vacuum distillation towers, followed by the separation of undesirable aromatic components and finally, by dewaxing and various finishing steps. Because aromatic components lead to high viscosity and extremely poor viscosity indices, the use of asphaltic type crudes is not preferred as the yield of acceptable lube stocks will be extremely low after the large quantities of aromatic components contained in such crudes have been separated out; paraffinic and naphthenic crude stocks will therefore be preferred but aromatic separation procedures will still be necessary in order to remove undesirable aromatic components. In the case of the lubricant distillate fractions, generally referred to as the neutrals, e.g. heavy neutral, light neutral, etc., the aromatics will be extracted by solvent extraction using a solvent such as phenol, furfural, N-methylpyrolidone or another material which is selective for the extraction of the aromatic components. If the lube stock is a residual lube stock, the asphaltenes will first be removed in a propane deasphalting (PDA) step followed by solvent extraction of residual aromatics to produce a lube generally referred to as bright stock.
The solvent extration to remove undesirable aromatic components is normally followed by a dewaxing step which is normally necessary in order for the lubricant to have a satisfactorily low pour point and a cloud point, so that it will not solidify or precipitate the less soluble paraffinic components under the influence of low temperatures. A number of dewaxing processes are known in the petroleum refining industry and of these, solvent dewaxing with solvents such as methylethylketone (MEK) and liquid propane, has been the one which has achieved the widest use in the industry. Recently, however, proposals have been made for using catalytic dewaxing processes for the production of lubricating oil stocks and these processes possess a number of advantages over the conventional solvent dewaxing procedures. The catalytic dewaxing processes which have been proposed are generally similar to those which have been proposed for dewaxing the middle distillate fractions such as heating oils, jet fuels and kerosenes, of which a number have been disclosed in the literature, for example, in Oil and Gas Journal, Jan. 6, 1975, pp. 69-73 and U.S. Pat. Nos. RE 28,398, 3,956,102 and 4,100,056. At least one of these processes, the Mobil lube Oil Dewaxing Process (MLDW) has now reached maturity and is capable of producing low pour point oils not attainable by solvent dewaxing. See 1986 Refining Process Handbook, Gulf Publishing Co., (Sept. 1986 Hydrocarbon Processing), page 90.
Generally, these catalytic dewaxing processes operate by selectively cracking the longer chain end paraffins to produce lower molecular weight products which may then be removed by distillation from the higher boiling lube stock. The catalysts which have been proposed for this purpose have usually been zeolites which have a pore size which admits the straight chain, waxy n-paraffins either alone or with only slightly branched chain paraffins but which exclude more highly branched materials and cycloaliphatics. Zeolites such as ZSM-5, ZSM-11, ZSM-12, ZSM-22, ZSM-23, and the synthetic ferrierites ZSM-35 and ZSM-38 have been proposed for this purpose in dewaxing processes, as described in U.S. Pat. Nos. 3,894,938, 4,176,050, 4,181,598, 4,222,855, 4,229,282 and 4,247,388. A dewaxing process employing synthetic offretite is described in U.S. Pat. No. 4,259,174. The relationship between zeolite structure and dewaxing properties is discussed in J. Catalysis 86, 24-31 (1984).
Although the catalytic dewaxing processes are commercially attractive because they do not produce quantities of solid paraffin wax which presently is regarded as an undesirable, low value product, they do have certain disadvantages and because of this, certain proposals have been made for combining the catalytic dewaxing processes with other processes in order to produce lube stocks of satisfactory properties. For example, U.S. Pat. No. 4,181,598 discloses a method for producing a high quality lube base stock by subjecting a waxy fraction to solvent refining, followed by catalytic dewaxing over ZSM-5 with subsequent hydrotreatment of the product. U.S. Pat. No. 4,428,819 discloses a process for improving the quality of catalytically dewaxed lube stocks by subjecting the catalytically dewaxed oil to a hydroisomerization process which removes residual quantities of petrolatum wax which contribute to poor performance in the Overnight Cloud Point test (ASTM D2500-66).
As mentioned above, the conventional catalytic dewaxing processes using intermediate pore size zeolites such as ZSM-5 operate by selectively cracking the waxy components of the feed. This results in a loss in yield since the components which are in the desired boiling range undergo a bulk conversion to lower boiling fractions which, although they may be useful in other products, must be removed from the lube stock. A notable advance in dewaxing process is described in U.S. Pat. Nos. 4,419,220 and 4,518,485, in which the waxy components of the feed, comprising straight chain and slightly branched chain paraffins, are removed by isomerization over a catalyst based on zeolite beta. During the isomerization, the waxy components are converted to relatively less waxy isoparaffins and at the same time, the slightly branched chain paraffins undergo isomerization to more highly branched aliphatics. A measure of cracking does take place during the operation so that not only is the pour point reduced by reason of the isomerization but, in addition, the heavy ends undergo some cracking or hydrocracking to form liquid range materials which contribute to a low viscosity product. The degree of cracking is, however, limited so as to maintain as much of the feedstock as possible in the desired boiling range. As mentioned above, this process uses a catalyst which is based on zeolite beta, together with a suitable hydrogenation-dehydrogenation component which is typically a base metal or a noble metal, usually of group VIA or VIIIA of the Periodic Table of the Elements (the periodic table used in this specification is the table approved by IUPAC), such as cobalt, molybdenum, nickel, tungsten, palladium or platinum. As described in U.S. Pat. No. 4,518,485, the isomerization dewaxing step may be proceded by a hydrotreating step in order to remove heteroatom-containing impurities, which may be separated in an interstage separation process similar to that employed in two-stage hydrotreating-hydrocracking processes.
With the present trend to more severe service ratings, there is a need to develop better lubricants. For example, the SAE service ratings of SD and SE are becoming obsolescent as more engine manufacturers specify an SF rating and it is expected that even more severe ratings will need to be met in the future as engine core temperatures increase in the movement toward greater engine efficiency. This progressive increase in service severity is manifested by improved resistance to oxidation at high temperatures and by higher V.I. requirements to ensure that the lubricants will have adequate viscosity at high temperatures without excessive viscosity when the engine is cold. In part, the improved performance may be obtained by improved additive technology but significant advances will be needed in basestock performance to accommodate more severe service requirements.
Because of their highly paraffinic nature, the waxes produced during conventional solvent dewaxing processes have been considered for use as lubestocks. Being highly paraffinic they have excellent V.I. but their high melting point generally precludes their use as automotive lubricants. Attempts have, however, been made to use them after suitable processing. The article by Bull in Developments in lubrication PD 19(2), 221-228 describes a process which subjects slack wax from a solvent (MEK-toluene) dewaxing unit to severe hydrotreating in a blocked operation together with other base stocks to produce high viscosity index (HVI) base oils. The promise of the process does not, however, appear to have been fully realized in practice since high V.I. oils of low pour point have not become commercially available. U.S. Pat. No. 4,547,283 describes a process for hydroisomerizing petroleum waxes such as slack wax using a specific type of catalyst treated with certain reactive metal compounds such as tetramethyl ammonium aluminate. Although high V.I. values are reported for the hydroisomerized wax products it is by no means clear that low pour points have been secured and accordingly it seem that the objective of matching low pour point with high V.I. in a lubricant of mineral oil origin has still to be met. A related proposal to use Foots Oil (the mixed oil/wax product of de-oiling slack wax) as a lube feedstock by dewaxing it over an intermediate pore size zeolite such as ZSM-5 is made in U.S. Pat. No. 3,960,705 but the products had relatively high pour points and the reported V.I. values do not exceed 107.
In Application Ser. No. 793,937 a process for producing high V.I., low pour point lubes from various paraffinic feeds such as slack wax or waxy gas oils such as the South East Asian gas oils is described. The process employs a first step in which a partial catalytic dewaxing is carried out with a zeolitic dewaxing catalyst which converts the waxy paraffin components is less waxy, high V.I. iso-paraffins. A subsequent, highly selective catalytic dewaxing is carried out using a highly shape selective dewaxing catalyst such as ZSM-23.
Ser. No. 044,187 describes lubricant products of extremely high quality which may be produced by a process of the type described in Application Ser. No. 793,937, using petroleum waxes as the feed. The lubricant products described here are characterized by high viscosity index (V.I.), low pour point (ASTM D-97) and retain their fluidity at low temperatures. These lubricants have a minimum V.I. of 130 and in most cases even higher values may be attained. Typical V.I. values are at least 140 and may even exceed 150 e.g., 155. The low temperature properties of the oils are outstanding:pour point (ASTM D-97) is no higher than 5.degree. F. (-5.degree. C.) and is typically below 0.degree. F. (about -18.degree. C.) and the Brookfield viscosity is less than 2500 P. at -20.degree. F. (about -29.degree. C.) for the basestock, i.e., additive-free stock. As manifested by the excellent high V.I., the relationship between temperature and viscosity is characterized by a relatively low decrease in viscosity with increasing temperature: at 40.degree. C., viscosity is typically no higher than 25 cSt. while at 100.degree. C. it is no less than 5.0 cSt and usually is higher e.g., 5.3 cSt.
These lubricants may be produced from petroleum waxes by a process of sequential hydroisomerization and hydrodewaxing as described in Ser. No. 793,937, followed by hydrotreating to remove residual aromatics and to stabilize the dewaxed product. Alternatively, the wax may first be deoiled to remove aromatics and the deoiled wax subjected to the hydroisomerization - hydrodewaxing sequence of Ser. No. 793,937 to produce the final lube base stock.
The lubricants described in Ser. No. 044,187 are highly paraffinic in nature by reason of their wax origin and because of this have a relatively low viscosity: most of the wax-derived products are of light neutral or medium neutral grade. It would be desirable to produce these high V.I., low pour point lubricants in higher viscosity grades e.g. corresponding to heavy neutral or bright stock without at the same time adversely affecting the viscosity index or pour point.
The use of peroxide treatment for modifying the viscosity of various lubestocks including distillates and hydrocracked resids has been described in U.S. Pat. Nos. 3,128,246 and 3,594,320. Other peroxide treatment processes are described in U.S. Pat. Nos. 4,594,172 and 4,618,737. Peroxide treatment has not, however, previously been proposed for use with wax-derived lubricants.
Peroxide treatment has been suggested for coupling or dimerization of Fischer-Tropsch paraffins. U.S. Pat. No. 4,594,172 (Sie) which is incorporated herein by reference, discloses converting synthesis gas to various paraffinic fractions by F-T. The C.sub.10 -C.sub.19 fraction, which was reported by the patentee to "consist(s) virtually completely of linear paraffins" was then given two peroxide treatments to couple the F-T wax, and produce a C.sub.20.sup.+ wax fraction.
The C.sub.20.sup.+ wax is then hydroisomerized over a Pt on Si-Al catalyst to produce a high V.I. lube oil.
The process provided a way to make high V.I. lube oils starting from synthesis gas, but there were some drawbacks. The process relied on F-T waxes, which have virtually no sulfur- and nitrogen compounds and cyclic compounds. This represents a specialized, and relatively expensive starting material, as compared to conventional hydrocarbon fractions derived from crude oil.
Extremely high consumptions of peroxide are required. Peroxidation experiment 6 in the patent required two treatments with 50 wt.% di-tert butyl peroxide to produce a higher molecular weight product.
Yields were also somewhat lower than desired. This is because the material started with wax, used DTBP treatment to couple the wax molecules and produce extremely long wax molecules which were still not suitable for use as a lubricating oil component. The long chain (C.sub.20.sup.+) waxy paraffins were given a hydroisomerization treatment to produce a product containing some hydroisomerized paraffins, which are high V.I. lube components. Oil yields of 21% by weight were achieved, based on the C.sub.20.sup.+ fraction used as a starting material.
Assuming that the peroxidation experiment proceeded with 100% efficiency, 100 g of C.sub.10 -C.sub.19 F-T wax would be converted to 100 g of C.sub.20.sup.+ wax, by the addition of 100 g of DTBP. The C.sub.20.sup.+ F-T waxes (obtained by peroxidation treatment) hydroisomerize to produce 21 g of oil product.
Although such a process can be used to produce an XHVI lube oil product from F-T waxes, it requires very large consumption of peroxide (100 g of peroxide were consumed to produce 21 g of product). It also required passing roughly five volumes of oil feed over a wax hydroisomerization unit to produce 1 volume of oil product (100 g of C.sub.20.sup.+ F-T wax was converted to 21 wt.% oil yield).
We believed there must be a more efficient way to produce an XHVI lube oil, without consuming such large amounts of peroxide relative to the volume of lube oil produced. We also wanted to minimize, if possible, the amount of material that had to be sent through a [wax hydroisomerization] peroxidation? unit, which can be a relatively expensive unit operation which can result in relatively low yields of oil product. We also wanted to develop a process which could be used not only on F-T waxes, but could also tolerate as a feedstock a waxy material derived from a paraffinic crude oil. Paraffinic crude oils contain small amounts of sulfur and nitrogen, so we wanted a process which could tolerate small amounts of impurities.